Magnetic resonance imaging (MRI) systems have become increasingly popular tools for medical diagnostics. Such systems are based on the influence of controlled magnetic fields on gyromagnetic material within a subject of interest. The gyromagnetic material, typically molecules such as water, have characteristic behaviors in response to external magnetic fields. The precession of spins of such molecules can be influenced by manipulation of such fields to obtain magnetic resonance emissions which can then be detected and processed to reconstruct useful images of the subject.
In MRI systems used in medical applications, a highly uniform primary magnetic field is produced by an electromagnet such that the field is generally aligned along the anatomy to be imaged. A series of gradient coils positioned around the subject produce additional magnetic fields used to select a desired slice of the anatomy to be imaged, and to encode positions of individual volume elements or voxels in the slice. Finally, a radio frequency coil is employed to pulse the gyromagnetic material to cause emissions from the material as it attempts to realign itself with the primary magnetic field. The emissions are sensed by a detection coil, which may be the same coil as the radio frequency coil. The sensed signals are then processed to identify the emissions emanating from specific locations of the voxels. A reconstructed image may be formed based upon the resulting data, wherein individual picture elements or pixels represent the voxels of the selected slice.
A wide variety of pulse sequences have been developed for MRI systems, and specifically adapted to various imaging needs. One motivating impetus behind a number of new imaging pulse sequences has been the reduction in the time required to acquire the image data. Such reductions in time are particularly useful for imaging moving tissues (both due to natural movement and movement of a subject patient), as well as for reducing the discomfort of patients in the examination process. For example, dramatic reductions in acquisition time have been obtained through a technique referred to as echo planar imaging (EPI). In this technique, successive data echoes corresponding to emissions from gyromagnetic material in a given data acquisition line are used to encode position information. The technique obtains rapid acquisition of data by providing image encoding from a train of successive data echoes within one relaxation interval rather than through phase encoding of separate lines of data with each relaxation interval as in other techniques.
Imaging sequences used in MRI systems, such as sequences used in EPI acquisition, employ bipolar readout gradients which oscillate in polarity many times during the readout phase of an examination. Such techniques often suffer from phase errors caused by misalignment of the data acquisition window with respect to the readout gradient pulse used to create a readout magnetic field. Other phase errors may result from eddy currents generated during the examination sequence, particularly by the readout gradients. Such eddy currents are created by rapidly changing drive currents used to generate the readout gradients, and tend to be more pronounced with stronger gradient fields and more rapid pulsing such as that used in EPI techniques. Several unwanted image artifacts can result from these phase errors, reducing the quality of the reconstructed image. Such artifacts include image ghosting, loss of image fidelity, and so forth. While certain phase errors can be corrected by estimating phase discrepancies between odd and even echoes in a readout sequence from a reference data set, such techniques are limited in their ability to accurately correct for the phase errors. Moreover, the reference data is typically acquired prior to an actual imaging sequence, adding substantially to the time required for the examination.
Moreover, the use of reference data for phase error compensation in bipolar readout gradient pulse sequences is still somewhat useful in addressing the phase error problem, for real-time interactive MRI examinations, a single reference scan is often insufficient. Real-time imaging involves continuous changes in scan orientation, and phase errors introduced in such examination sequences depend upon the scanned plane. Consequently, new phase errors cannot typically be corrected using phase data derived from a single reference scan acquired in a different scan orientation. Although a new reference scan can be acquired at each scan location, imaging efficiency is further reduced by such additional reference scans, owing to the fact that such reference scans typically are non-phase encoded and so contain no useful image information. Moreover, since most reference data is obtained without the application of phase-encoding gradients, the data cannot account for phase errors contributed by the phase-encoding gradients used in actual imaging sequences.
There is a need, therefore, for an improved technique for correcting or compensating for potential phase errors in bipolar readout gradient acquisition sequences in MRI systems. There is a particular need for a technique which does not significantly add to the data acquisition or processing time required, and which provides effective compensation for phase errors closely associated with the individual lines of data acquired during an examination sequence.